This add-on is not compatible with your version of Thunderbird because of the following:

Version 0.3
Released September 23, 2007
8.2 kB
Works with
Thunderbird 2.0 - 3.0a3

2007-08-04: xSearchbar v0.3:

* Added translations from Babelzilla.* Added a default preference for the localized description.* Added a skin directory and used some CSS to make the height of the view picker closer to that of the searchbox.

2007-06-05: xSearchbar v0.2:

* Added the View->Toolbars->Search Bar menu item.* Remove the Views and QuickSearch toolbar buttons from the main toolbox palette so it doesn't show up in the Customize window.